The present invention generally relates to optical systems, and relates more particularly to focusing lens configurations used in infrared imaging systems having a cooled detector array.
Forward looking infrared imaging systems (or "FLIRS") using high performance semiconductor detector materials usually require cryogenic cooling in order to increase detector sensitivity. In these systems, optimum performance is achieved by "cold shielding" the detector array; i.e., by introducing a cold diaphragm in front of the detector array so that the viewing angle of the detectors to the warm background is limited as far as possible to that required for transmission of the radiation from the scene. Since the radiation to the detector array from outside of this viewing angle is emitted from the cold diaphragm, it is generally negligible, so that the condition of minimum background radiation, and, hence, minimum background noise is achieved.
In first generation infrared imaging systems, the detection system generally either comprises a long column of detectors and one dimensional scanning ("parallel scan") or one or two rows of a few detectors each and two dimensional scanning ("serial scan"). In the case of the former, cold shielding is accomplished by means of a narrow slit in conjunction with a mask of individual detector cold shields placed near the focal plane. In the latter case, the detector array is small so that effective cold shielding can be accomplished by means of a cold diaphragm placed as far as convenient from the detector array. In either system the detector array is mounted in a cryogenic vessel (or "dewar") to achieve optimum performance, and the imaging lens of the system is positioned outside the dewar.
The new (second) generation of FLIRS utilizes a detector array abutting a large charge coupled device (CCD) in conjunction with scanning optics in order to provide improved imaging in two dimensions. This type of system raises additional design constraints. First, the imaging lens (or "detector lens") of the system must be corrected for the wider field of view presented by the detector array as a result of its larger size. Adequate correction usually requires more than one optical element, even if an aspheric element is utilized. Second, the detector array cannot be efficiently cold shielded because a diaphragm in close proximity to the detector array must be large in order to avoid obscuring the field of view of any of the detector elements in the array.
Because two or more optical elements are generally required in a wide field of view system and these are typically outside the dewar, either the size of the scanning optics will be increased because they are farther away from the aperture stop (or its image) of the system, or cold shielding efficiency will be degraded because the aperture stop of the system will be outside the dewar, and consequently warm. Thus, for such wide field of view systems, the aperture stop is ideally placed outside the dewar from the standpoint of minimizing scanner size and achieving aberration correction. On the other hand, for optimum cold shielding the aperture stop should be cold, and, therefore, placed inside the dewar.
It is accordingly a primary object of the present invention to provide an improved optical system for thermal imaging systems using wide field of view optics in conjunction with large detector arrays.